Electrically insulative objects and ungrounded metallic objects tend to acquire charges of static electricity which may range up to several thousand volts. Charge accumulation results from several causes such as movement and the accompanying friction, induction and receipt of discharges from other objects or from charged surfaces.
The eventual discharge of accumulations of static electricity can have undesirable effects and in some circumstances can cause severe damage to objects such as certain industrial products. A notable example occurs in the manufacture of miniaturized semiconductor electronic components. Static discharges can destroy the minute conductive paths in integrated circuit wafers, microchips and the like, and have been an important cause of the high rejection rate of such products during the manufacturing process. Static charges also attract and cause adherence of dust particles and other contaminants that can adversely affect the product.
Manufacture of such products is performed in areas termed clean rooms in which elaborate precautions are taken to eliminate potential contaminants and also to suppress electrostatic charge buildup on the products. Maintaining a high level of free ions in the air which surrounds the product is one of the more effective techniques for suppressing such charge buildup. Positive and negative ions of the constituent gases of air are electrostatically attracted to charge accumulations of opposite polarity and then neutralize such accumulations by charge exchange.
The conventional air ionizer for such purposes includes one or more high voltage electrodes which are typically situated several feet away from the objects that are to be protected. The intense electrical field created by the electrode causes a corona discharge and acts to dissociate molecules of the constituent gases of air into charged ions. Ions having a polarity similar to that of the electrode are repelled by the electrode and disburse outwardly towards the products which are to be protected. Electrodes of both polarities are provided or the voltage on a single electrode is periodically reversed in order to generate ions of both polarities. The system must be more or less continuously monitored and adjustments made as needed to assure that the appropriate ratio of positive to negative ions is maintained. An imbalance, which may occur from such causes as unequal electrode erosion, can have the counter-productive effect of imparting charge to the products.
The conventional air ionizing apparatus and procedures are not satisfactorily compatible with recent developments in clean room technology which include more closely controlling the environment of the products. Efforts are being made to reduce the level of particulate contamination in the atmosphere which is adjacent to the product. In some cases these include maintaining the products in isolation boxes, to the extent possible, during processing. The boxes are continuously purged with a flow of very clean inert gas such as nitrogen. Modern clean rooms commonly operate at particulate levels of fewer than 100 particles per cubic foot and some operate at fewer than 10 particles per cubic foot.
High voltage air ionizing apparatus must necessarily be spaced a substantial distance from the products to allow for intermixing of the positive and negative ions which are produced at spaced apart electrodes or at alternating time periods at the same electrode. If the electrodes are too close, the apparatus may itself impart charge to the products. Thus such apparatus cannot be placed inside isolation boxes or the like unless they are of excessive size.
The effective range of the conventional system is undesirably limited under many working conditions. Ions of opposite polarity continually neutralize each other while drifting from the electrode to the products which are to be protected. Ions of either polarity are also electrostatically attracted to walls or other nearby objects and are then neutralized by charge exchange. Thus the ion content in the air falls rapidly as a function of distance from the ionizing electrodes. This problem cannot be cured by locating the high voltage electrodes in close proximity to the products. As previously discussed, that can cause an imparting of static charge to the products rather neutralization of charge.
Further, the conventional high voltage air ionizing apparatus has itself been found to be a source of particulate contamination at levels that can be significant where an extremely clean product environment is needed.
In particular, such apparatus releases metallic particles into the adjacent atmosphere which typically have a size around 300 Angstrom units. This is believed to result from erosion of the high voltage electrodes by the corona discharges which occur at the electrodes. Heat, sputtering and the presence of free radicals in the discharge may be contributing factors. In any case, particle release is a demonstrable occurence which can be minimized by use of special electrode materials but which cannot be entirely eliminated.
A further problem is encountered in that high voltage discharges may convert some atmospheric oxygen into ozone. Ozone is a highly reactive gas which can be very damaging to certain products such as the semiconductor wafers discussed above.
The background of the invention has been herein discussed with reference to the suppression of electrostatic charge accumulations on objects. There are also other reasons why it may be beneficial to provide an ionized atmosphere at a particular region such as, for example, air purification. A high ion content in the air at a particular region acts to remove dust, smoke, pollens and other particulates from the air. The particulates acquire an electrical charge by charge exchange with such ions and are then electrostatically attracted to nearby walls or other surfaces.
The present invention is directed to overcoming one or more problems discussed above.